Developmental constraints control generalist invertebrate distributions across a gradient of unpredictable disturbance

Abstract

Mechanisms underpinning flexible life-history strategies have rarely been tested in hydrologically unpredictable ecosystems where generalists may have life-history trade-offs and developmental constraints that limit their distributions. We investigated in situ nymphal growth and developmental strategies of 2 generalists, Xanthocnemis zealandica and Sigara arguta, across a habitat-permanence gradient. In response to temporary pond drying, we anticipated a flexible generalist response with rapid growth and shorter development, resulting in smaller adult size. In comparison, we expected nymphs living in permanent lakes with predatory fish to extend growth and development in favor of larger adult size. Both species maximized growth rates in temporary ponds but had different developmental strategies that influenced their distribution. Xanthocnemis zealandica had longer development requirements (≥125 d), which limited their distribution in less predictable temporary ponds, whereas S. arguta were less constrained in development (≥56 d) and inhabited more temporary ponds. The longer development time of X. zealandica meant they benefited from flexible life-history traits: shorter development, limited desiccation tolerance in temporary ponds, and extended development and predator avoidance in permanent habitats. Sigara arguta had an opportunistic life-history strategy with a fixed, rapid developmental response across the permanence gradient and rapid colonization of refilled temporary ponds. This fixed strategy meant S. arguta was intolerant of drying and, in permanent lakes, was found only in shallow refuges from fish. Neither species differed in adult size across the permanence gradient. Our study shows how life-history strategies enable generalists to achieve broad distributions in a heterogeneous waterscape, and that resilience and flexibility to local selection pressures depend on the constraints of their phenologies.